1. Field of the Invention
The invention relates to a method for a purifying a hydrocarbon wash liquid from the de-ashing of an olefin polymer prepared using TiCl.sub.3 and an organoaluminum compound as catalyst.
2. Description of the Prior Art
The polymerization of 1-olefins by contact with so-called Ziegler or Ziegler-Natta catalysts comprising essentially titanium trichloride and an aluminum alkyl compound is a well developed art. The olefin of prime commercial interest in this type of process is propylene. Although the present invention is applicable in the work-up of any of the known Ziegler polymers of 1-olefins, including, for example, polyethylene, poly(4-methyl-1-pentane) and ethylene-propylene copolymers, it will be discussed primarily in terms of polypropylene work-up. Commercial processes for production of polypropylene predominantly employ a liquid hydrocarbon medium of higher boiling point than propylene as diluent in the polymerization reaction. It is also known to conduct the polymerization in a medium consisting of propylene which may contain minor amounts of propane and possible small amounts of other hydrocarbon components.
The crude propylene polymers obtained in this way contain catalyst residues which may adversely affect the properties of the polymer. By "catalyst residues" is meant the catalyst and any decomposition products or derivatives thereof which may be formed during the polymerization.
In addition to catalyst residues, the polymerization reaction product also contains some hydrocarbon-soluble propylene polymer components, usually referred to as atactic polypropylenes, which it may be desirable to remove in whole or in part prior to recovery of the desired propylene polymer product.
Many different methods have been disclosed in the art for treating propylene polymerization reaction product for deactivation of catalyst components, removal of deactivated catalyst components and removal of undesired atactic polymer. A number of different methods are employed in commercial propylene polymerization processes.
The procedure whereby catalyst residues are removed from the polymer is usually termed "de-ashing". In general the de-ashing comprises two steps. First the catalyst residues are solubilized by treatment, for example, with oxygen and/or certain organic compounds such as alcohols or dicarbonyl compounds. The second step involves washing the polymer with a suitable wash liquid, for example, a hydrocarbon, to remove the solubilized residues. An example of a suitable de-ashing procedure is to be found in Netherlands patent application No. 7,600,033, published July 9, 1976.
Most of the usual solubilization steps result in the liberation of hydrogen chloride, which may give rise to corrosion problems in process vessels and conduits, or require the use of expensive corrosion-resistant equipment. It has been proposed to prevent corrosion difficulties due to HCl liberation by adding during the solubilization step an alkylene oxide, such as propylene oxide, which reacts with the HCl evolved for form a chlorohydrin. Part of this chlorohydrin tends to remain adsorbed on the treated polymer particles, even after washing the latter with a liquid hydrocarbon. The presence of alkylene oxide and/or chlorohydrin in the polymer product is undesirable, especially if articles are manufactured from such polymer which will come into contact with food.
Another disadvantage of the use of relatively volatile alkylene oxides, such as propylene oxide, is that in the fractionation step which is part of the work-up of the dirty hydrocarbon wash liquid separated from the polymer product, the alkylene oxide may contaminate the clean diluent or the monomer which are recovered for recirculation to the polymerization reactor. This would interfere with the catalytic polymerization activity of the TiCl.sub.3 and organoaluminum compound. Moreover, because of the volatility of the propylene oxide, it will be substantially absent in the heavier component stream of the fractionator containing the amorphous polymer and catalyst residues, as a result of which corrosion may occur in the subsequent work-up apparatus.